Electronic devices, such as communication devices and home entertainment devices, include an audio sub-system for providing an audio output. The audio sub-system generally includes an input device to provide audio signals, an audio amplifier to amplify the audio signals received, and one or more speakers to produce an amplified sound. Audio amplifiers used for amplifying audio signals are generally classified into various classes including A, B, AB, C, and D. Class-D amplifiers, in particular, have an advantage of providing high-efficiency audio output at minimum power loss and are, thus, preferred in applications where power efficiency is important, such as in battery powered and portable applications.
A class-D amplifier is a switching amplifier and typically includes an integrator, a modulation circuit, and a driver circuit in a feedback configuration. The integrator pre-amplifies a feedback error signal and provides a pre-amplified audio signal to one or more comparators of a modulation circuit. The modulation circuit converts the amplified audio signal into a pulse width modulated (PWM) signal using a variety of schemes known in the art. Further, the PWM signal is fed to the driver circuit, which provides an amplified audio signal to the speakers.
The various components of the class-D amplifier, such as the integrator, the comparators, and the driver circuit are generally implemented using operational amplifiers. However, operational amplifiers often generate voltage offset due to fabrication imperfections. Moreover, as the components are generally small in size, it is difficult to achieve appropriate matching between the components. As a result, in certain conditions such as powering-up of the electronic device, some spurious signals are introduced at the inputs of the speaker. Such spurious signals amount to a pop-up noise in the speaker during powering-up or un-muting of the electronic device. Typically, the pop-up noise caused due to the mismatch in the driver circuit is reduced by pre-biasing the driver circuit while enabling a normal operating mode of the speakers. However, pre-biasing the driver circuit does not reduce the pop-up noise caused by the offsets in the integrator or the comparators.